Photomultiplier tubes are adversely affected by exposure to high light levels while operating. In many cases, the physical mechanisms responsible for these effects are not understood. The effects result in a temporary loss in sensitivity and a temporary increase in dark current. These temporary effects can persist for many hours after exposure to an intense light source. It has been determined experimentally that reducing the tube gain by reverse biasing the photocathode dramatically reduces these effects.
In U.S. Pat. No. 4,436,994 to James G. Van Vliet, and James R. Brown assigned to Beckman Instruments, Inc. of Fullerton, Calif. entitled PHOTOMULTIPLIER DETECTOR PROTECTION DEVICE AND METHOD a photomultiplier detection device is disclosed. The photomultiplier in Van Vliet utilizes a negative feedback loop to maintain a photomultiplier detector below a predetermined output. The feedback loop in Van Vliet comprises a comparator which responds to the photomultiplier output. The output of the photomultiplier is limited by a signal generated by a digital to analog converter. The voltage control signal controls the output of a power supply which is applied to the photomultiplier detector. The output of the photomultiplier detector is modulated by a high voltage power supply which is in turn modulated by a digital to analog converter using a counter as a set voltage. Van Vliet senses the output of the photomultiplier tube and not the number of photons incident upon the photomultiplier tube. Also, Van Vliet adjusts the main power supply voltage and does not adjust the voltage of the focus electrode separately. Van Vliet also does not provide a frequency to voltage converter which would be needed if incident photon frequency is measured. The photomultiplier detector protection device and method disclosed in Van Vliet depends heavily on the ability to sense a preamplified output of the photomultiplier which has fully multiplied the incoming radiation to generate an output voltage. The protection scheme of Van Vliet cannot work using photon counting electronics.